fix: serve post-robot.js locally

[maureendu]

My organization's users have experienced a persistent issue with our Custom Tab, because of how post-robot.js is served to the app. For those users who are affected, the Custom Tab remains blank. A call that loads post-robot.js can be seen in browser debugging tools.

By trying to load this file over the web, a call is made to https://cdn.rawgit.com/krakenjs/post-robot/7da06445/dist/post-robot.js and this call has been consistently failing for some of our users (likely depending on their geographic location).

This is a 3rd party dependency, i.e. not something that Bullhorn owns, so instead of relying on the file being served over the web, we fixed the issue by serving the post-robot.js file locally.